The present invention relates generally to optical communication systems, and particularly to a method and apparatus for compensating for chromatic dispersion in optical systems.
Optical transmission systems, including optical fiber communication systems, have become an attractive vehicle for carrying voice and data at high speeds. In optical transmission systems waveform degradation due to chromatic dispersion in the optical transmission medium can be problematic, particularly as transmission speeds are ever-increasing. Chromatic dispersion results from the fact that in transmission media such a glass optical fibers, the higher the frequency of the optical signal, the greater the refractive index. As such, higher frequency components of optical signals will xe2x80x9cslow down,xe2x80x9d and contrastingly, lower frequency signals will xe2x80x9cspeed up.xe2x80x9d Moreover, impurities in the glass mechanical stress and strain, and temperature effects can also affect the index of refraction, further adding to the ill-effects of chromatic dispersion.
In digital optical communication systems, where the optical signal is ideally a square wave, bit spreading due to chromatic dispersion can be particularly problematic. As the xe2x80x9cfast frequenciesxe2x80x9d in the signal slow down and the xe2x80x9cslow frequenciesxe2x80x9d in the signal speed up as a result of dispersion, the shape of the wave can be substantially impacted. Accordingly, the effects of this type of dispersion are a spreading of the original pulse in time, causing it to overflow in the time slot that has been allotted to another bit. When the overflow becomes excessive, inter-symbol interference (ISI) may occur. ISI may result in an increase in bit-error rate to unacceptable levels.
An example of bit spreading due to chromatic dispersion is shown in FIG. 1. The xe2x80x9c101xe2x80x9d bit sequence shown at 101, 102, 103, respectively, is an ideal signal transmission in this illustrative example. As can be readily understood by one of ordinary skill in the art, in order to achieve sharp rising edges 104 and the sharp trailing edges 105 a large number of relatively high frequency Fourier components is required. As described briefly above, these higher frequency components are xe2x80x9cslowed downxe2x80x9d through chromatic dispersion, and as can be seen in signal 106, the waveform is significantly altered (spread) due to the dispersion of the higher frequency components. Because the peak power in each pulse is reduced as the total power is spread out, and power is distributed between bits as a result of dispersion, predetermined detection thresholds used to discriminate digital bit xe2x80x9c1""sxe2x80x9d from digital bit xe2x80x9c0""sxe2x80x9d may not be attained. Thus, the chromatic dispersion may create unacceptable bit-error rates as the xe2x80x9c1""sxe2x80x9d have insufficient power to reach a detection threshold, and the xe2x80x9c0""sxe2x80x9d have too much power, and thereby exceed the threshold.
Conventional techniques to curb the ill-effects of chromatic dispersion are primarily static in nature. One conventional static technique includes the use of chromatic dispersion (CD) compensating fiber in which positive or negative dispersion can be selectively introduced into an optical communication system. For example, if positive chromatic dispersion is present, negative compensating optical fiber may be selectively placed in the optical communication system to xe2x80x9creshapexe2x80x9d the spread signal.
Another static technique to curb the ill-effects of chromatic dispersion includes the use of dispersion shifted fiber, which is designed exhibit no chromatic dispersion at a particular wavelength. Unfortunately, in a particular optical communication system, there may be approximately 40 channels or more, having channel center wavelengths spaced at approximately 0.8 nm to approximately 1 nm increments. Illustratively, a 40 channel system could have channel center wavelengths in the range of approximately 1530 nm to 1570 nm. At the 1550 nm wavelength channel, there is little or no chromatic dispersion, barring other influences. However, in channels having shorter center channel wavelengths, positive chromatic dispersion is introduced; while in channels having longer channel center wavelengths negative chromatic dispersion is introduced. Accordingly, compensation for chromatic dispersion is not readily realized across the entire 40 channel system using this static CD compensation technique.
While the above described techniques have resulted in some success in compensating for chromatic dispersion, these solutions cannot account for time dependent changes in the optical system. Moreover, these solutions require a prior knowledge of the chromatic dispersion of a system. Therefore, the conventional static solutions are neither capable of handling dynamic changes in the chromatic dispersion of an optical signal nor are these solutions capable of ready adaptation for use in a variety of optical systems.
Accordingly, what is needed is a method and apparatus for compensating chromatic dispersion in optical networks that overcomes the shortcomings of the static chromatic dispersion techniques discussed above.
According to an exemplary embodiment of the present invention, an optical device for detecting chromatic dispersion in an optical system is disclosed. The optical device includes a receiver, which converts an optical signal into an electrical signal having a plurality of frequency components. A bandpass section separates the plurality of frequency components of the electrical signal. Each of the frequency components has a corresponding voltage level. The optical device also includes a gain section which amplifies each of the corresponding voltage levels of the plurality of frequency components, wherein the device outputs a measure of chromatic dispersion in the optical signal based on a comparison of a power level of each of the frequency components to that of an ideal optical signal with no chromatic dispersion.
According to another exemplary embodiment, an apparatus for compensating for chromatic dispersion in an optical signal is disclosed. The apparatus includes a detector, which converts at least a portion of the optical signal into an electrical signal having a plurality of frequency components; a controller, which receives an output from the detector, wherein the detector outputs a measure of the chromatic dispersion present in the optical signal based on a comparison of a power level of each of said frequency components to that of an ideal optical signal with no chromatic dispersion; and a dispersion compensator, which introduces corrective chromatic dispersion based on input from the controller.
According to yet another exemplary embodiment of the present invention, a method for compensating for chromatic dispersion is disclosed. The illustrative method includes converting a portion of an optical signal into an electrical signal having a plurality of frequency components; comparing each of the frequency components of the electrical signal with those of an ideal optical signal having no chromatic dispersion; and introducing corrective chromatic dispersion into the optical signal based on the comparing, so that chromatic dispersion is substantially zero in the official signal.